This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-167550, filed Jun. 14, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a digital-signal quality monitoring method which is used in a digital transmission system such as an optical transmission system and can easily monitor the quality of transmission signals; and a communications apparatus which uses this method.
In the field of recent digital transmission systems, as improvements are being made on the wave-length multiplexing technology and optical switching technology, there is an expectation for realization of large-capacity and highly-flexible optical networks. A great attention is paid particularly to a WDM (Wavelength Division Multiplexing) network which is characterized by its capability of employing different transmission speeds, transmission frame formats and modulation systems for different wavelength channels.
This type of network needs such control as to always monitor the quality of signals to transfer in an optical fiber and switch that optical fiber to a redundant system when some failure occurs in the optical fiber. Possible factors for the degradation of the quality of signals in an optical fiber are the deterioration of the S/N ratio caused by an increase in spontaneous emission (ASE) noise that is generated in an optical amplifier, the deterioration of waveforms originated by the wavelength dispersion and the non-linear characteristic of a transmission fiber and crosstalk to another wavelength channel due to a variation in signal wavelength.
A Q value has often been used as a parameter for monitoring the quality of binary digital signals that travel through an optical fiber. The Q value is a parameter which represents the S/N ratio that is defined by the following equation.   Q  =            "LeftBracketingBar"                        μ          1                -                  μ          0                    "RightBracketingBar"                      σ        1            +              σ        0            
where xcexc1 and xcexc0 are arverage values of the mark level and space level respectively and "sgr"1 and "sgr"0 are dispersion values of the mark level and space level respectively.
Available methods for the acquisition of a Q value are a method of acquiring a Q value by measuring the reception error ratio while scanning a discrimination threshold value, as proposed in IEEE Photonics Tecnol. Lett., Vol. 5, No. 3, pp. 304-306, 1993, xe2x80x9cMargin measurements in optical amplifier systemxe2x80x9d by N. S. Bargano et al., and a method of acquiring a Q value from the amplitude histogram of an eye pattern obtained by using the sampling technique, as proposed in ECOC ""98, pp. 707-709, xe2x80x9cApplication of amplitude histograms for quality of service measurements of optical channels and fault identificationxe2x80x9d by K. Muller et al.
But, the first quality monitoring method which evaluates the quality by acquiring a Q value from a measured error ratio needs to generate a reference data pattern from a signal to be measured in order to detect an error, and also needs to count error bits. The second quality monitoring method which evaluates the quality by acquiring a Q value from the amplitude histogram of an eye pattern obtained by sampling requires a sampling circuit like a sampling oscillo-scope and a data processing circuit. The use of those quality monitoring methods results in an increased number of constituting components and enlarges the apparatus. Those methods also complicate the measuring algorithm.
While an error in a received signal can be measured by using error monitoring bytes B1 and B2 in an STM-n (Synchronous Transport Module-n) frame, this method not only requires a frame process but also a long time measure the signal quality when the error ratio is very small.
Accordingly, it is an object of the present invention to provide a digital-signal quality monitoring method which can easily and accurately monitor the quality of received digital signals, and a communications apparatus which uses this method.
To achieve the above object, a digital-signal quality monitoring method according to this invention is designed as follows.
(1) A digital-signal quality monitoring method comprising the steps of scanning a discrimination level while discriminating a value of an input n-value digital signal (n being a natural number equal to or larger than 2) by comparing that value with the discrimination level; detecting an average value of the discriminated signal; and computing a quality parameter from the average value.
(2) A digital-signal quality monitoring method comprising the steps of scanning a discrimination level and discrimination timings while discriminating a value of an input n-value digital signal (n being a natural number equal to or larger than 2) by comparing that value with the discrimination level; detecting an average value of the discriminated signal; and computing a quality parameter from the average value.
(3) In the digital-signal quality monitoring method (1), a probability density function along an amplitude axis of the input signal is acquired by differentiating the average value of the discriminated signal with respect to the discrimination levels and the quality parameter is computed from the probability density function.
(4) In the digital-signal quality monitoring method (2), a probability density function along an amplitude axis of the input signal at individual discrimination timing is acquired by differentiating the average value of the discriminated signal with respect to the discrimination levels and the quality parameter is computed from the probability density function.
(5) In the digital-signal quality monitoring method (1), a probability density function along an amplitude axis of the input signal is acquired by scanning the discrimination levels in such a way as to be proportional to time and differentiating the average value of the discriminated signal with respect to time and the quality parameter is computed from the probability density function.
(6) In the digital-signal quality monitoring method (2), a probability density function along an amplitude axis of the input signal at individual discrimination timing is acquired by scanning the discrimination levels in such a way as to be proportional to time and differentiating the average value of the discriminated signal with respect to time and the quality parameter is computed from the probability density function.
(7) In any one of the digital-signal quality monitoring methods (3) to (6), a probability density function along an amplitude axis with no input made has been acquired in advance, and the probability density function acquired at a time a signal is input is corrected based on that former probability density function.
(8) In any one of the digital-signal quality monitoring methods (3) to (6), an n number of average values and n number of dispersion values of the discriminated signal are acquired from the probability density function and the quality parameter is computed from the probability density function.
(9) In the digital-signal quality monitoring method (4) or (6), an eye opening is computed from the probability density function along the amplitude axis of the input signal at individual discrimination timing.
The following are feasible communications apparatuses which use the digital-signal quality monitoring method of this invention.
(10) A regular-system/reserved-system switching apparatus comprising switching means for selectively outputting an n-value digital signal (n being a natural number equal to or larger than 2) of a regular system or an n-value digital signal of a reserved system; quality monitoring means for monitoring the n-value digital signal output by the switching means, thereby acquiring a quality parameter; switching control means for controlling a switching action of the switching means based on the quality parameter obtained by the quality monitoring means, whereby the quality monitoring means scans a discrimination level while discriminating a value of the n-value digital signal by comparing that value with the discrimination level, detects an average value of the discriminated signal and computes a quality parameter of the n-value digital signal from the average value.
(11) A regular-system/reserved-system switching apparatus comprising quality monitoring means for monitoring n-value (n being a natural number equal to or larger than 2) digital signals of both a regular system and a reserved system and acquiring quality parameters; switching means for selectively outputting the n-value digital signal of the regular system or the n-value digital signal of the reserved system; and switching control means for controlling a switching action of the switching means based on the quality parameters of the regular system and the reserved system obtained by the quality monitoring means, whereby the quality monitoring means scans a discrimination level while discriminating values of the n-value digital signals of both the regular system and the reserved system by comparing those values with the discrimination level, detects average values of the discriminated signals and computes quality parameters of the n-value digital signals of both the regular system and the reserved system from the average values.
(12) In the regular-system/reserved-system switching apparatus (10) or (11), the quality monitoring means scans the discrimination levels and also discrimination timings.
(13) A transmission quality monitoring apparatus for monitoring transmission qualities of n-value digital signals (n being a natural number equal to or larger than 2) in a plurality of wavelength channels, which are optically transmitted over an optical transmission line of a wavelength division multiplexing network, comprising optical branching means for branching a part of transmitted light from the optical transmission line of the wavelength division multiplexing network; digital signal extracting means for extracting digital signals of the plurality of wavelength channels from the transmitted light branched by the optical branching means; quality monitoring means for receiving the digital signals of the plurality of wavelength channels extracted by the digital signal extracting means and acquiring quality parameters of the digital signals; and check means for checking if there is an abnormality for each of the wavelength channels from the quality parameters obtained by the quality monitoring means and generating an abnormality detection signal upon detection of the abnormality, whereby the quality monitoring means scans a discrimination level while discriminating values of the n-value digital signals of the wavelength channels by comparing those values with the discrimination level, detects average values of the discriminated signals and computes quality parameters for the wavelength channels from the average values.
(14) A transmission quality monitoring apparatus for monitoring transmission qualities of n-value digital signals (n being a natural number equal to or larger than 2) in a plurality of wavelength channels, which are optically transmitted over an optical transmission line of a wavelength division multiplexing network, comprising optical branching means for branching a part of transmitted light from the optical transmission line of the wavelength division multiplexing network; digital signal extracting means for extracting digital signals of the plurality of wavelength channels from the transmitted light branched by the optical branching means; signal selecting means for selectively outputting an arbitrary one of the digital signals of the plurality of wavelength channels extracted by the digital signal extracting means; quality monitoring means for receiving the digital signal selected by the signal selecting means and acquiring a quality parameter of the digital signal; and check means for checking if there is an abnormality in the selected digital signal from the quality parameter obtained by the quality monitoring means and generating an abnormality detection signal upon detection of the abnormality, whereby the quality monitoring means scans discrimination level while discriminating a value of the digital signal selected by the signal selecting means by comparing that value with the discrimination level, detects an average value of the discriminated signal and computes a quality parameter for each of the wavelength channels from the average value.
(15) In the transmission quality monitoring apparatus (13) or (14), the quality monitoring means scans the discrimination levels and also discrimination timings.
(16) A receiving apparatus for demodulating a data signal by comparing an input n-value digital signal (n being a natural number equal to or larger than 2) with (nxe2x88x921) threshold values, comprising quality monitoring means for scanning discrimination levels while discriminating a value of the input digital signal by comparing that value with the discrimination levels, detecting an average value of the discriminated signal, and computing a quality parameter from the average value; and threshold value control means for detecting (nxe2x88x921) discrimination levels that optimize the quality parameter acquired by the quality monitoring means and using the (nxe2x88x921) discrimination levels as the (nxe2x88x921) threshold values.
(17) A receiving apparatus for demodulating a data signal by comparing an input n-value digital signal (n being a natural number equal to or larger than 2) with (nxe2x88x921) threshold values, comprising quality monitoring means for scanning discrimination levels and discrimination timings while discriminating a value of the input digital signal by comparing that value with the discrimination levels, detecting an average value of the discriminated signal, and computing a quality parameter from the average value; and threshold value control means for detecting (nxe2x88x921) discrimination levels that optimize the quality parameter acquired by the quality monitoring means and using the (nxe2x88x921) discrimination levels as the (nxe2x88x921) threshold values.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.